Profiles the given file or expression using Erlang's eprof tool.

:eprof provides time information of each function call and can be useful when you want to discover the bottlenecks related to this.

Before running the code, it invokes the app.start task which compiles and loads your project. After that, the target expression is profiled together with all matching function calls using the Erlang trace BIFs. The tracing of the function calls for that is enabled when the profiling is begun, and disabled when profiling is stopped.

To profile the code, you can use syntax similar to the mix run task:

$ mix profile.eprof -e Hello.world
$ mix profile.eprof -e "[1, 2, 3] |> Enum.reverse |> Enum.map(&Integer.to_string/1)"
$ mix profile.eprof my_script.exs arg1 arg2 arg3

This task is automatically re-enabled, so you can profile multiple times in the same Mix invocation.

Command line options

• --matching - only profile calls matching the given Module.function/arity pattern
• --calls - filters out any results with a call count lower than this
• --time - filters out any results that took lower than specified (in µs)
• --sort - sorts the results by time or calls (default: time)
• --eval, -e - evaluates the given code
• --require, -r - requires pattern before running the command
• --parallel, -p - makes all requires parallel
• --no-warmup - skips the warmup step before profiling
• --no-compile - does not compile even if files require compilation
• --no-deps-check - does not check dependencies
• --no-archives-check - does not check archives
• --no-halt - does not halt the system after running the command
• --no-start - does not start applications after compilation
• --no-elixir-version-check - does not check the Elixir version from mix.exs

Profile output

Example output:

#                                               CALLS     % TIME µS/CALL
Total                                              24 100.0   26    1.08
Enum.reduce_range_inc/4                             5  3.85    1    0.20
:erlang.make_fun/3                                  1  7.69    2    2.00
Enum.each/2                                         1  7.69    2    2.00
anonymous fn/0 in :elixir_compiler_0.__FILE__/1     1  7.69    2    2.00
:erlang.integer_to_binary/1                         5 15.39    4    0.80
:erlang.apply/2                                     1 15.39    4    4.00
anonymous fn/3 in Enum.each/2                       5 19.23    5    1.00
String.Chars.Integer.to_string/1                    5 23.08    6    1.20

Profile done over 8 matching functions

The default output contains data gathered from all matching functions. The first row after the header contains the sums of the partial results and the average time for all the function calls listed. The following rows contain the function call, followed by the number of times that the function was called, then by the percentage of time that the call uses, then the total time for that function in microseconds, and, finally, the average time per call in microseconds.

When --matching option is specified, call count tracing will be started only for the functions matching the given pattern:

#                                               CALLS     % TIME µS/CALL
Total                                               5 100.0    6    1.20
String.Chars.Integer.to_string/1                    5 100.0    6    1.20

Profile done over 1 matching functions

The pattern can be a module name, such as String to count all calls to that module, a call without arity, such as String.split, to count all calls to that function regardless of arity, or a call with arity, such as String.split/3, to count all calls to that exact module, function and arity.

Caveats

You should be aware that the code being profiled is running in an anonymous function which is invoked by :eprof module. Thus, you'll see some additional entries in your profile output. It is also important to note that the profiler is stopped as soon as the code has finished running, and this may need special attention, when: running asynchronous code as function calls which were called before the profiler stopped will not be counted; running synchronous code as long running computations and a profiler without a proper MFA trace pattern or filter may lead to a result set which is difficult to comprehend.

You should expect a slowdown in your code execution using this tool since :eprof has some performance impact on the execution, but the impact is considerably lower than Mix.Tasks.Profile.Fprof. If you have a large system try to profile a limited scenario or focus on the main modules or processes. Another alternative is to use Mix.Tasks.Profile.Cprof that uses :cprof and has a low performance degradation effect.